In general, the treatment of an inorganic salt containing an organic material discharged from plants is subject to difference in treatment method or conditions used due to difference in the kind and concentration of organic material in inorganic salt and the kind and discharged amount of inorganic salt or difference in the target level of treatment.
For example, in order to treat an inorganic salt containing an organic material as it is in the form of powder, it has been tried to subject the inorganic salt powder to heat treatment on a fluidized bed, a moving bed (shaft kiln) or a rotary kiln. However, it is known that when an inorganic salt containing an organic material is subjected to heat treatment on a fluidized bed, the inorganic salt powder undergo agglomeration to form coarse particle even at temperatures lower than the melting point thereof, disabling the operation (“Kogyo Hannou Souchi” (Industrial Reaction Apparatus), Baifukan, page. 147, Feb. 25, 1984). Similarly, it is known that when an inorganic salt powder is subjected to heat treatment on a rotary kiln, the inorganic salt powder is attached to the wall of the rotary kiln at temperatures lower than the melting point thereof during the heat treatment of the inorganic salt powder, causing scaling that disables the operation. Accordingly, it is made difficult to subject an inorganic salt powder containing an organic material to heat treatment on an industrial basis.
As other treatment methods there have been attempted various methods which comprise dissolving an inorganic salt containing an organic material in water, and then selectively removing the organic material therefrom.
For example, JP-A-2-90985 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a method which comprises dissolving an inorganic salt containing an organic material in water, and then adsorbing the organic material by an activated carbon column as a fixed bed. It is certain that the method proposed in the above cited patent can remove an organic material by adsorption. However, when the adsorption continues, the activated carbon is saturated with the organic material, making the treatment insufficient. Accordingly, in order to remove the organic material in a stable manner, it is necessary that adsorption and desorption be repeated, complicating the operation.
Another proposal is a biological treatment method which comprises dissolving an inorganic salt containing an organic material in water, and then converting the organic material to a harmless material such as carbon dioxide gas and water via activated sludge. This method is advantageous in environmental protection. However, when the concentration of inorganic salt in the aqueous solution increases, the activated sludge can no longer be alive, requiring that the operation be carried out at a lowered concentration of inorganic salt and hence raising the amount of the liquid to be treated. Further, the treatment by activated sludge proceeds at a reduced rate and requires the use of large-sized facilities that need a large installation area. Moreover, it is necessary that the conditions optimum for the living of activated sludge be maintained, complicating the operation control.
A further proposal for treatment method is a chemical treatment method which comprises adding an oxidizing agent to an aqueous solution of inorganic salt containing an organic material so that the organic material is converted to an environmentally harmless material such as carbon dioxide gas and water. The treatment method thus proposed can be operated at a raised rate by raising the temperature or the concentration of oxidizing agent. However, the treatment method is disadvantageous in that it requires the addition of a chemical in an amount substantially proportional to the amount of the organic material. Thus, the treatment of an inorganic salt containing an organic material in a high concentration adds to cost. Further, it is difficult to control the amount of the oxidizing agent to be added according to the variation of the concentration of the organic material. Thus, this proposal is impractical.